J. Physiol. (1979), 288, pp. 401-410 With 2 text-figurew Printed in Great Britain
401
SURAL NERVE EFFECTS ON MEDIAL GASTROCNEMIUS MOTONEURONES IN THE CAT
BY J. G. COLEBATCH AND J. D. GILLIES From the School of Medicine, University of N.S. W., Kensington, N.S.W. 2033, Australia
(Received 18 July 1978) SUMMARY
1. Excitability cycles for medial gastrocnemius motoneurones were recorded following sural nerve stimuli with amplitudes of 1-5, 3, 5, 10 and 40 times the nerve threshold. The monosynaptic reflex was time-integrated to quantify the motoneuronal output and some implications of this technique are discussed. In particular it is calculated that this method specifically examines the 15-25 % most excitable motoneurones of the medial gastrocnemius pool. 2. In most cases high amplitude stimulation of the sural nerve caused a triphasic (excitatory-inhibitory-excitatory) change in excitability. Arguments are given to support the conclusion that this represents a corresponding post-synaptic alteration in medial gastrocnemius motoneurone potential. It is concluded that this pattern illustrates the nature of the sural nerve projections to this portion of the medial gastrocnemius pool. 3. The first period of excitation began after a latency, corrected to be compatible with intracellular recordings, of 2-8 ms and had a mean duration of 4-0 ms. The minimum stimulus level necessary for this effect lay between 1-5 and 3 times the nerve threshold. The maximum amplitude of this facilitation occurred with stimuli of 3-5 times threshold and was 60-90 % of the amplitude of the monosynaptic facilitation which followed stimulation of the lateral gastrocnemius-soleus nerve. 4. A period of inhibition followed immediately after the period of excitation and had a mean duration of 16 ms. The minimum stimulus level necessary for this effect lay between 1-5 and 3 times the nerve threshold and the degree of inhibition increased with stimuli up to 40 times threshold. 5. The second period of excitation lasted a mean period of 50 ms and was due to activity in high threshold fibres. On average its amplitude was 50 % that of the initial excitation. 6. Exceptions to this triphasic variation of excitability were found. This result is interpreted as indicating the presence of projections with opposing actions which were simultaneously activated by sural stimuli. INTRODUCTION
Activity in cutaneous afferents contributes to the flexion reflex (Sherrington, 1910; Lloyd, 1943; Eccles & Lundberg, 1959), a finding that led Holmqvist & Lundberg (1961) to include them in their 'flexion reflex afferents' category. Less well
J. G. COLEBATCH A ND J. D. GILL IES known, however, is that even in the early literature (e.g. Sherrington, 1910, 1947) and supported by scattered but consistent reports of more recent years, there is evidence that segmental cutaneous input can also lead to excitation of extensor motoneurones (Creed, Denny-Brown, Eccles, Liddell & Sherrington, 1932; Bernhard, 1947; Hagbarth & Naess, 1950; Hagbarth, 1952; Wilson, 1963; Engberg, 1964; Burke, Jankowska & ten Bruggencate, 1970). Experiments performed on walking cats also support this conclusion (Forssberg, Grillner & Rossignol, 1975; Duysens & Pearson, 1976). Bernard (1947) showed long latency (20-100 ms) facilitation with preceding inhibition in medial gastrocnemius from sural volleys. Hagbarth & Naess (1950) confirmed this finding and also showed that repetitive supramaximal stimuli produced long lasting facilitation which was abolished by a small dose of dial. Wilson (1963) and Burke et al. (1970) have shown short latency facilitation. To elucidate further the properties of the projection of the sural nerve onto medial gastrocnemius, we have studied the effects of single sural volleys of various amplitudes on the medial gastrocnemius motoneurone pool and related the changes in excitability to those following a heteronymous volley in the lateral gastrocnemius-soleus nerve. A preliminary account of this work has been published (Colebatch & Gillies, 1977). 402
METHODS
Eleven adult cats were anaesthetized with a mixture of alpha chloralose (40 mg/kg) and urethane (200 mg/kg) given intravenously. Supplementary anaesthetic was rarely required. Conventional surgical procedures were used to expose the lumbosacral spinal cord, transact the spinal cord at the L1-L3 level and form paraffin oil pools over the exposed cord and popliteal fossa area of the right leg. Body temperature was regulated at 37 + 1 'C. The cord and leg pool temperatures stablized near 35 CC. The sural nerve and the nerves to medial gastrocnemius and lateral gastrocinemius-soleus muscles were isolated, cut and the proximal ends used for bipolar stimulation. The L7 and SI ventral roots were also freed, cut and the proximal ends then mounted for bipolar recording; in most experiments records were made from the S1 root alone. Nerve thresholds were determined by observing the potential recorded by an electrode at the site of entry of the L7 dorsal root fibres. The test medial gastrocnemius reflex was produced by a single electrical stimulus of 0-2 ms duration, repeated at 1/s and applied to the medial gastrocnemius nerve at three times the level for threshold excitation. A preceding conditioning stimulus to the medial gastrocnemius nerve was not used because it was found that this (subliminal) volley could discharge medial gastroenemius motoneurones during periods of facilitation, and these motoneurones were then refractory to the following test stimulus. The sural volleys were evoked at 1/s by a single electrical stimulus of 0-2 msec duration, and the nerve threshold determined. Six different stimulus amplitudes were used (1-5, 3, 5, 10 and 40 times the nerve threshold). The monosynaptic medial gastrocnemius reflex was recorded from the proximal cut end of the L7 or 51 ventral root and the time integral of the reflex discharge was measured with a gated electronic integrator. The period of integration was restricted to the duration of the monosynaptic discharge (mean 1-5 ms, range 11-1 8 ms). The time integral of a monosynaptic reflex has been shown to be a more useful measure of its size than its amplitude (Clamann, Gillies, Skinner & Henneman, 1974). The integrated signal was led to a storage oscilloscope and the pulse used to initiate and terminate the period of integration intensified the oscilloscope beam. This resulted in a pair of stored points separated by a vertical distance wvhich was proportional to the area of the monosynaptic reflex. In some animals recordings were also made from the distal end of the cut ventral nerve root. A complete excitability cycle consisted of 50 or 100 successive reflex responses. The time relationship between the stimuli to the medial gastrocnemius and sural nerves was varied and the reflex repeated so that the area of reflexes which occurred immediately prior to and 10,
MOTONEURONAL EXCITABILITY
403
50 or 100 ms following the sural stimulus were recorded. A single excitability cycle therefore consisted of 50 or 100 consecutive points. The time between the two stimuli was altered so that the cycle was built up in an interlaced manner; thus, for example, in a 100 point series the 1st, the 11th, the 21st,...91st points would be gathered and then the 2nd, the 12th, the 22nd and so on until 100 reflex areas were stored. As a result of this procedure juxtaposed points were based on observations made 10 s apart. This ensured that slow changes in the average excitability of the medial gastrocnemius motoneurone pool or of the la terminals which might have occurred over the 100 s required to record a full cycle would cause only an increased scatter in the line of points. Recordings with excessive variability were rejected. The excitability of the preparation was usually such that the sural nerve volleys themselves caused a small polysnaptic discharge in the ventral root. When the monosynaptic test reflex occurred during such discharge the integrator output represented the combined areas of the reflex and the polysynaptic discharge occurring during the period of integration. When this happened the excitability cycle was repeated using the sural stimulus alone and the resulting control cycle was then subtracted from the test cycle. The excitability cycle technique gives longer latencies for post-synaptic effects than those derived from intracellular recordings. This discrepancy is largely due to the finite time (about 0-5 ms) between the onset of depolarization and the discharge of motoneurones (Araki, Eccles & Ito, 1960). To make our results compatible with data derived from intracellular recordings the effect of a single volley in the lateral gastrocnemius-soleus nerve on medial gastrocnemius excitability was recorded and the latency of all sural effects was measured relative to the onset of this monosynaptic facilitatory effect. To calculate the absolute latency from cord entry of sural effects on medial gastrocnemius motoneurones, 0 5 ms was added to the latency as measured above to allow for the mean lateral gastrocnemius-soleus intracord conduction time and a single synaptic delay at medial gastrocnemius motoneurones (Brock, Coombs & Eccles, 1952; see also Fig. 11 of Araki et al. 1960). This corrected figure has been used in all results. RESULTS
In order to determine the percentage of the medial gastrocnemius motoneurone
pool activated in these experiments, the area of the antidromic medial gastrocnemius volley was measured in the distal end of the cut ventral root (Clamann, Gillies & Henneman, 1974). The area of the control reflex was 10-15% of the area of the antidromic volley which, allowing for the non-linearity introduced by the different fibre diameters (see Discussion), indicates that the unconditioned reflexes involved 15-25 % of the most excitable motoneurones of the medial gastrocnemius pool. These results are based on excitability cycles for the medial gastrocnemius mono-
synaptic reflex of eleven cats, conditioned by sural stimuli with amplitudes of 1-5 (8 cycles), 3 (8 cycles), 5 (8 cycles), 10 (5 cycles) and 40 (5 cycles) times the nerve threshold. The ipsilateral motor cortex had been removed several months prior to the experiment in six cats and in the remainder the nervous system was intact prior to the acute spinalization. The results were similar for both types preparation and have been presented as a single group. The usual change in medial gastrocnemius reflex excitability following strong sural shocks (10-40 times threshold) was triphasic; this consisted of an early period of facilitation followed by inhibition of a similar magnitude and finally a prolonged facilitation of smaller amplitude than the earlier effects. This pattern was observed in 5 out of 7 excitability cycles. At lower stimulus strengths (1.5-5 times threshold) only the earlier diphasic changes were found (9 out of 10 cycles). The early sural faciliation was first present following stimuli of 1-5-3 times the nerve threshold and was maximal with stimuli 3-5 times threshold (Fig. 1). At its peak, this effect increased the amplitude of the medial gastrocnemius reflex by a
J. G. COLEBATCH AND J. D. GILLIES mean of 86 % (range: 37-120 %). In each animal the relative effectiveness of this early facilitation was compared with the maximum facilitation following a single stimulus applied to the nerve to lateral gastrocnemius-soleus. The maximum amplitude of the early sural facilitation was from 60-90 % of the monosynaptic facilitation obtainable from lateral gastrocnemius-soleus. Its mean latency from cord entry was 2*8 ms (range: 2*5-3*1 ms). The excitatory effect lasted a mean period of 4'0 ms 404
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405 MOTONEURONAL EXCITABILITY (range: 2*9-4*9 ms) after which a period of inhibition followed (mean duration 16 ms, range: 9-24 ms). This inhibitory effect involved fibres first recruited by stimuli with amplitudes between 1*3 and 3 times the nerve threshold but the maximum effect was not observed until stimulation levels 40 times threshold were used. In all cases this inhibition was, with stimulation at 40 times threshold strength, sufficient at its peak to abolish completely the monosynaptic medial gastrocnemius reflex. The later excitation, which followed the period of inhibition, was seen with sural stimuli of 10-40 times the nerve threshold. The effect was always smaller than the earlier facilitation (mean reflex increase, 43 %, range: 15-83 %) but more prolonged (mean duration, 50 ms, range: 40-70 ms). Triphasic excitability changes were not invariably found. Two cats showed different patterns of excitability change. In one, the latency of the inhibitory phase was increased and inhibition did not follow activity in low threshold fibres. At the latency when inhibition usually began, there was in this animal a second peak of facilitation (Fig. 2). This facilitation first appeared with stimulus levels of 1P5 times the nerve threshold but did not reach its peak until stimuli of 10 times threshold were used. This preparation also displayed unusually pronounced flexor responses following sural volleys and had marked ankle clonus. The second atypical preparation showed a contrasting pattern. Late excitation was absent and the phase of inhibition was unusually prolonged with a duration of 29 ms. The motor cortex of this cat had been removed 2 months prior to the experiment, but since these changes were not found in the other five similar preparations this unusual pattern could not be attributed to the antecedent lesion. DISCUSSION
Single volleys in the sural nerve produced a sequence of excitability changes in medial gastrocnemius. Sural afferents recruited by stimuli with amplitudes of less than 1*5 times the nerve threshold caused no excitability changes; this 'ineffectiveness', at a segmental level, of activity in the largest fibres of the sural nerve has been previously reported (Rosenberg, 1970). Afferents recruited by stimuli with Fig. 1. An example of the changes in medial gastrocnemius excitability most commonly found after sural nerve stimulation. Each point represents one observation of integrated reflex amplitude. The upper panel of A shows the triphasic excitability change which followed high amplitude (40 times threshold (T)) stimulation of the sural nerve. The control, immediately below, indicates the discharge which was evoked by the sural stimulus alone and must be subtracted from the upper panel to obtain the actual medial gastrocnemius excitability changes. B shows more detail of the early changes (note faster time base). A sural stimulus with an amplitude 1-5 times nerve threshold level caused no alteration in medial gastrocnemius excitability but a stimulus of 3 times nerve threshold caused a 4 ms period of excitation as well as later inhibition which is present at the end of the record. Control records (not reproduced here) indicated that these sural volleys alone evoked no discharge. The arrows show the time when monosynaptic excitation began after stimulation of the nerve to lateral gastrocnemius-soleus and are used to calculate the latencies of the effects (see text). Some points have been retouched.
J. G. COLEBATCH AND J. D. GILLIES 406 amplitudes of between 1P5 and 3 times the nerve threshold have both excitatory and inhibitory effects. The earliest segmental effect of sural stimuli is facilitation, with a latency (mean, 2-8 ms from cord entry) consistent with a trisynaptic linkage. High threshold (10-40 times nerve threshold) fibres also have both inhibitory and, later, excitatory actions. Despite the high stimulus levels, the latency of even the .
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407 MOTONEURONAL EXCITABILITY late effects is consistent with myelinated fibre activity. With the exception of the lowest threshold fibres, afferents with excitatory and inhibitory projections are present throughout the myelinated fibre spectrum of the sural nerve. The motoneurones of a motor pool appear to have a relatively fixed order of recruitment in response to changes in synaptic excitation (Henneman, Somjen & Carpenter, 1965). If this is the case, the first recruited must be the cells most used by the animal during movements. Ths makes the reflex connections of these cells of particular interest. However, these cells are also the smallest present in the pool which means that, for medial gastrocnemius, their diameters approximate those of large y motoneurones (Burke, Strick, Kanda, Kim & Walmsley, 1977). Intracellular study of these small motoneurones is likely to be almost as difficult as it is for y motoneurones, which are only occasionally entered and deteriorate rapidly (Eccles, Eccles, Iggo & Lundberg, 1960). The excitability cycle technique, which consists of recording the change in recruitment following an homonymous la volley, offers a simple method to examine specifically the excitability changes of a population of these small cells. Results obtained with this technique may therefore differ from commonly reported intracellular records. However, since the voltage generated by an axon is proportional to the square of its diameter (Clamann & Henneman, 1976), there is not a linear relationship between the percentage of the motor pool discharged and the amplitude of the integrated reflex. As a consequence the percentage increase in the integrated reflex amplitude is greater than the percentage of new motoneurones recruited. It is possible to calculate, given the fibre distribution reported by Boyd & Davey (1968), that the control reflex involved 15-25 % of the total pool and that to double its amplitude required only about a 70 % increase in the number of fibres discharging. Although well suited to studying this part of the motoneurone pool, excitability changes shown with this technique may not occur post-synaptically. The size of the monosynaptic reflex is a measure of the excitability of the pathway from the la fibres to the motoneurones, so that changes in the excitability of either of these two elements will cause alterations in the size of the test reflex. Evidence that these changes do occur post-synaptically comes from intracellular recordings from medial gastrocnemius motoneurones. These have shown that both early and late depolarizations, as well as hyperpolarization occur in some medial gastrocnemius motoneurones following sural stimuli (Wilson, 1963; Engberg, 1964; Burke et al. 1970; Fig. 2. An example of an exception to the more common diphasic pattern of the early excitability changes following sural volleys. In this animal stimulation of the sural nerve at two times the threshold level (T) (caused an initial peak of excitation followed by a
further period of facilitation. This later period of facilitation contrasted with the more frequent presence of inhibition following a short initial period of excitation (Fig. 1). An increase in the stimulus intensity to 10 times the nerve threshold caused the early period of facilitation to be increased slightly (after the control discharge is subtracted) whereas the following excitation is markedly increased. These findings suggest that a facilitatory pathway, more potent in this animal, is present in addition to the inhibitory one which usually is dominant. The arrows indicate the time when facilitation following stimulation of the lateral gastrocnemius-soleus nerve began on comparable records and are used to derive latencies (see text). Some of the points have been retouched.
J. G. COLEBATCH AND J. D. GILLIES Rosenberg, 1970; see below). Hyperpolarization of la terminals follows sural volleys with a latency similar to that of the late period of facilitation (Rudomin, Nunez, Madrid & Burke, 1974). A contribution from this source is unlikely, however, as the fibres responsible for the hyperpolarization have a low threshold, unlike those 408
responsible for the late facilitation described here. Facilitatory effects of sural volleys on extensor motoneurones have been shown previously. Bernhard (1947), using the excitability cycle technique, showed a period of facilitation following inhibition of gastrocnemius after sural volleys. The latency of this change was compatible with the late facilitation shown here. No early facilitation was noted. Wilson (1963) found, with intracellular recording from gastrocnemius-soleus cells, that sural volleys frequently caused short latency e.p.s.p.s. These latencies ranged from 2*5 to 5 ms and the thresholds of the fibres responsible ranged from 1-2 to 2*2 times the nerve threshold. He also found late depolarizations in response to stimuli of much higher amplitudes. Our results are in general agreement with these observations, as our briefer latencies would be biased towards the earliest facilitation occurring in the group of motoneurones. Our results are also consistent with those of Burke et al. (1970) who examined the nature of the sural projection to different parts of the triceps surae motor pool. Half of the motoneurones innervating type S motor units, which in general have the smallest sizes (Burke, 1967), had early e.p.s.p.s followed by a later i.p.s.p. An additional advantage of recording reflex amplitude changes to establish neural connections is that an estimate of the relative effectiveness of two projections to a population of motoneurones can be made, unaffected by the sampling bias of intracellular recording. The early facilitation was found to augment the reflex area by 60-90 % of that caused by a heteronymous la volley from lateral gastrocnemiussoleus. The early facilitatory sural projection must therefore be regarded both as a potent and widespread segmental facilitatory input to medial gastrocnemius motoneurones. Cutaneous fibres were included among the 'flexion reflex afferents' grouping of Holmqvist & Lundberg (1961). Their inclusion was not entirely consistent with earlier observations (Hagbarth, 1952) and subsequent evidence derived from intracellular recordings (Wilson, 1963; Engberg, 1964; Hongo, Jankowska & Lundberg, 1966, 1969) led to the separation of low threshold cutaneous fibres from the 'flexion reflex afferents', which continued to include high threshold fibres (Hongo et al. 1969). Our results for the sural projection indicate that high threshold fibres, as well as low threshold fibres, have excitatory actions in addition to the accepted inhibitory effects. These excitability changes are not necessarily typical of other cutaneous projections, although Wilson (1963) also found late depolarizations after stimulating the superficial peroneal nerve. The early facilitatory change, attributed to relatively low threshold sural afferents, was always present in these experiments. In contrast, the later inhibitory and excitatory phases were not seen in every animal, and were sometimes replaced by activity of the opposite type. These observations suggest that these later phases normally represent the net result of both excitatory and inhibitory projections converging upon these motoneurones and either acting simultaneously or alternately. Such parallel pathways have been shown for projections to flexor motoneurones
409 MOTONEURONAL EXCITABILITY (Holmqvist & Lundberg, 1961) as well as in walking cats (Forssberg et al. 1975; Duysens & Pearson, 1976). This arrangement would also explain the change from late facilitation to inhibition following the administration of dial anaesthetic (Hagbarth & Naess, 1950). In these medial gastrocnemius motoneurones, the typical excitability changes following sural volleys is a triphasic one, with inhibition preceded and followed by facilitation. The early period of facilitation at least has such a potency and is so widespread that further investigation should attempt to delineate in more detail its characteristics and role in movement. Future investigation should also attempt to show if these changes are typical of this part of the motor pool of other extensor muscles following activity in cutaneous nerves. REFERENCES
ARAKI, T., ECCLES, J. C. & ITO, M. (1960). Correlation of the inhibitory post-synaptic potential of motoneurones with the latency and time course of monosynaptic reflexes. J. Physiol. 154, 354-377. BERNHARD, C. G. (1947). Slow cord potentials correlated to reciprocal functions. Acta physiol. scand. 14, suppl. 47, 6. BOYD, I. A. & DAVEY, M. R. (1968). Composition of Peripheral Nerves, p. 28. Edinburgh: Livingstone. BROCK, L. G., COOMBS, J. S. & ECCLES, J. C. (1952). The recording of potentials from motoneurones with an intracellular electrode. J. Physiol. 117, 431-460. BURKE, R. E. (1967). Motor unit types of cat triceps surae muscle. J. Physiol. 193, 141-160. BURKE, R. E., JANKOWSKA, E. & TEN BRUGGENCATE, G. (1970). A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae. J. Physiol. 207, 709-732. BURKE, R. E., STRICK, P. L., KANDA, K., KIM, C. C. & WALMSLEY, B. (1977). Anatomy of medial gastrocnemius and soleus motor nuclei in cat spinal cord. J. Neurophysiol. 40, 667-680. CLAMANN, H. P., GILLIES, J. D. & HENNEMAN, E. (1974). Effects of inhibitory inputs on critical firing level and rank order of motoneurones. J. Neurophysiol. 37, 1350-1360. CLAMANN, H. P., GILLIES, J. D., SKINNER, R. D. & HENNEMAN, E. (1974). Quantitative measures of output of a motoneurone pool during monosynaptic reflexes. J. Neurophysiol. 39, 1328-1337. CLAMANN, H. P. & HENNEMAN, E. (1976). Electrical measurement of axon diameter and its use in relating motoneuron size to critical firing level. J. Neurophysiol. 39, 844-851. COLEBATCH, J. G. & GILLIES, J. D. (1977). Effects on cat medial gastrocnemius motoneurones of segmental input from the sural nerve. Proceedings of the Australian Physiological and Pharmacological Society. 8, 56P. CREED, R. S., DENNY-BROWN, D., ECCLES, J. C., LIDDELL, E. G. T. & SHERRINGTON, C. S. (1932). Reflex Activity of the Spinal Cord, repr. 1972. Oxford: Oxford University Press. DUYSENS, J. & PEARSON, K. G. (1976). The role of cutaneous afferents from the distal hindlimb in the regulation of the step cycle of thalamic cats. Exp. Brain Res. 24, 245-255. ECCLES, J. C., ECCLES, R. M., IGGO, A. & LUNDBERG, A. (1960). Electrophysiological studies on gamma motoneurones. Acta physiol. scand. 50, 32-40. ECCLES, R. M. & LUNDBERG, A. (1959). Synaptic actions in motoneurones by afferents which may evoke the flexion reflex. Archs ital. Biol. 97, 199-221. ENGBERG, I. (1964). Reflexes to foot muscles in the cat. Acta physiol. scand. 62, Suppl. 235. FORSSBERG, H., GRILLNER, S. & ROSSIGNOL, S. (1975). Phase dependent reflex reversal during walking in chronic spinal cats. Brain Res. 85, 103-107. HAGBARTH, K.-E. (1952). Excitatory and inhibitory skin areas for flexor and extensor motoneurones. Acta physiol. scand. 26, supply. 94. HAGBARTH, K.-E. & NAESS, K. (1950). Reflex effects of tetanic stimulation of different afferent fibre-systems in the hind limb of the cat. Acta physiol. scand. 21, 336-361. HENNEMAN, E., SOMJEN, G. & CARPENTER, D. 0. (1965). Excitability and inhibitibility of motoneurons of different sizes. J. Neurophysiol. 28, 599-620.
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HOLMQVIST, B. & LUNDBERG, A. (1961). Differential supraspinal control of synaptic actions evoked by volleys in the flexion reflex afferents in alpha motoneurones. Acta phy8iol. 8cand. 54, suppl. 186. HONGO, T., JA~xowsKA, E. & LUNDBERG, A. (1966). Convergence of excitatory and inhibitory action on interneurones in the lumbosacral cord. Exp. Brain Res. 1, 338-358. HoNGo, T., JwowsKA, E. & LUNDBERG, A. (1969). The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones. Exp. Brain Res. 7, 365-391. LLOYD, D. P. C. (1943). Neuron patterns controlling transmission of ipsilateral hind limb reflexes in cat. J. Neurophysiol. 6, 293-315. ROSENBERG, M. E. (1970). Synaptic connexions of alpha extensor motoneurones with ipsilateral and contralateral cutaneous nerves. J. Physiol. 207, 231-255. RUDOMIN, P., NuNEz, R., MADRID, J. & BURKE, R. E. (1974). Primary afferent hyperpolarisation and presynaptic facilitation of la afferent terminals induced by large cutaneous fibers. J. Neurophysiol. 37, 413-429. SHERRINGTON, C. S. (1910). Flexion-reflex of the limb, crossed extension-reflex, and reflex stepping and standing. J. Physiol. 40, 28-121. SHERRINGTON, C. S. (1947). The Integrative Action of the Nervous System, 2nd edn. New Haven: Yale University Press. WILSON, V. J. (1963). Ipsilateral excitation of extensor motoneurones. Nature, Lond. 198, 290291.
401
SURAL NERVE EFFECTS ON MEDIAL GASTROCNEMIUS MOTONEURONES IN THE CAT
BY J. G. COLEBATCH AND J. D. GILLIES From the School of Medicine, University of N.S. W., Kensington, N.S.W. 2033, Australia
(Received 18 July 1978) SUMMARY
1. Excitability cycles for medial gastrocnemius motoneurones were recorded following sural nerve stimuli with amplitudes of 1-5, 3, 5, 10 and 40 times the nerve threshold. The monosynaptic reflex was time-integrated to quantify the motoneuronal output and some implications of this technique are discussed. In particular it is calculated that this method specifically examines the 15-25 % most excitable motoneurones of the medial gastrocnemius pool. 2. In most cases high amplitude stimulation of the sural nerve caused a triphasic (excitatory-inhibitory-excitatory) change in excitability. Arguments are given to support the conclusion that this represents a corresponding post-synaptic alteration in medial gastrocnemius motoneurone potential. It is concluded that this pattern illustrates the nature of the sural nerve projections to this portion of the medial gastrocnemius pool. 3. The first period of excitation began after a latency, corrected to be compatible with intracellular recordings, of 2-8 ms and had a mean duration of 4-0 ms. The minimum stimulus level necessary for this effect lay between 1-5 and 3 times the nerve threshold. The maximum amplitude of this facilitation occurred with stimuli of 3-5 times threshold and was 60-90 % of the amplitude of the monosynaptic facilitation which followed stimulation of the lateral gastrocnemius-soleus nerve. 4. A period of inhibition followed immediately after the period of excitation and had a mean duration of 16 ms. The minimum stimulus level necessary for this effect lay between 1-5 and 3 times the nerve threshold and the degree of inhibition increased with stimuli up to 40 times threshold. 5. The second period of excitation lasted a mean period of 50 ms and was due to activity in high threshold fibres. On average its amplitude was 50 % that of the initial excitation. 6. Exceptions to this triphasic variation of excitability were found. This result is interpreted as indicating the presence of projections with opposing actions which were simultaneously activated by sural stimuli. INTRODUCTION
Activity in cutaneous afferents contributes to the flexion reflex (Sherrington, 1910; Lloyd, 1943; Eccles & Lundberg, 1959), a finding that led Holmqvist & Lundberg (1961) to include them in their 'flexion reflex afferents' category. Less well
J. G. COLEBATCH A ND J. D. GILL IES known, however, is that even in the early literature (e.g. Sherrington, 1910, 1947) and supported by scattered but consistent reports of more recent years, there is evidence that segmental cutaneous input can also lead to excitation of extensor motoneurones (Creed, Denny-Brown, Eccles, Liddell & Sherrington, 1932; Bernhard, 1947; Hagbarth & Naess, 1950; Hagbarth, 1952; Wilson, 1963; Engberg, 1964; Burke, Jankowska & ten Bruggencate, 1970). Experiments performed on walking cats also support this conclusion (Forssberg, Grillner & Rossignol, 1975; Duysens & Pearson, 1976). Bernard (1947) showed long latency (20-100 ms) facilitation with preceding inhibition in medial gastrocnemius from sural volleys. Hagbarth & Naess (1950) confirmed this finding and also showed that repetitive supramaximal stimuli produced long lasting facilitation which was abolished by a small dose of dial. Wilson (1963) and Burke et al. (1970) have shown short latency facilitation. To elucidate further the properties of the projection of the sural nerve onto medial gastrocnemius, we have studied the effects of single sural volleys of various amplitudes on the medial gastrocnemius motoneurone pool and related the changes in excitability to those following a heteronymous volley in the lateral gastrocnemius-soleus nerve. A preliminary account of this work has been published (Colebatch & Gillies, 1977). 402
METHODS
Eleven adult cats were anaesthetized with a mixture of alpha chloralose (40 mg/kg) and urethane (200 mg/kg) given intravenously. Supplementary anaesthetic was rarely required. Conventional surgical procedures were used to expose the lumbosacral spinal cord, transact the spinal cord at the L1-L3 level and form paraffin oil pools over the exposed cord and popliteal fossa area of the right leg. Body temperature was regulated at 37 + 1 'C. The cord and leg pool temperatures stablized near 35 CC. The sural nerve and the nerves to medial gastrocnemius and lateral gastrocinemius-soleus muscles were isolated, cut and the proximal ends used for bipolar stimulation. The L7 and SI ventral roots were also freed, cut and the proximal ends then mounted for bipolar recording; in most experiments records were made from the S1 root alone. Nerve thresholds were determined by observing the potential recorded by an electrode at the site of entry of the L7 dorsal root fibres. The test medial gastrocnemius reflex was produced by a single electrical stimulus of 0-2 ms duration, repeated at 1/s and applied to the medial gastrocnemius nerve at three times the level for threshold excitation. A preceding conditioning stimulus to the medial gastrocnemius nerve was not used because it was found that this (subliminal) volley could discharge medial gastroenemius motoneurones during periods of facilitation, and these motoneurones were then refractory to the following test stimulus. The sural volleys were evoked at 1/s by a single electrical stimulus of 0-2 msec duration, and the nerve threshold determined. Six different stimulus amplitudes were used (1-5, 3, 5, 10 and 40 times the nerve threshold). The monosynaptic medial gastrocnemius reflex was recorded from the proximal cut end of the L7 or 51 ventral root and the time integral of the reflex discharge was measured with a gated electronic integrator. The period of integration was restricted to the duration of the monosynaptic discharge (mean 1-5 ms, range 11-1 8 ms). The time integral of a monosynaptic reflex has been shown to be a more useful measure of its size than its amplitude (Clamann, Gillies, Skinner & Henneman, 1974). The integrated signal was led to a storage oscilloscope and the pulse used to initiate and terminate the period of integration intensified the oscilloscope beam. This resulted in a pair of stored points separated by a vertical distance wvhich was proportional to the area of the monosynaptic reflex. In some animals recordings were also made from the distal end of the cut ventral nerve root. A complete excitability cycle consisted of 50 or 100 successive reflex responses. The time relationship between the stimuli to the medial gastrocnemius and sural nerves was varied and the reflex repeated so that the area of reflexes which occurred immediately prior to and 10,
MOTONEURONAL EXCITABILITY
403
50 or 100 ms following the sural stimulus were recorded. A single excitability cycle therefore consisted of 50 or 100 consecutive points. The time between the two stimuli was altered so that the cycle was built up in an interlaced manner; thus, for example, in a 100 point series the 1st, the 11th, the 21st,...91st points would be gathered and then the 2nd, the 12th, the 22nd and so on until 100 reflex areas were stored. As a result of this procedure juxtaposed points were based on observations made 10 s apart. This ensured that slow changes in the average excitability of the medial gastrocnemius motoneurone pool or of the la terminals which might have occurred over the 100 s required to record a full cycle would cause only an increased scatter in the line of points. Recordings with excessive variability were rejected. The excitability of the preparation was usually such that the sural nerve volleys themselves caused a small polysnaptic discharge in the ventral root. When the monosynaptic test reflex occurred during such discharge the integrator output represented the combined areas of the reflex and the polysynaptic discharge occurring during the period of integration. When this happened the excitability cycle was repeated using the sural stimulus alone and the resulting control cycle was then subtracted from the test cycle. The excitability cycle technique gives longer latencies for post-synaptic effects than those derived from intracellular recordings. This discrepancy is largely due to the finite time (about 0-5 ms) between the onset of depolarization and the discharge of motoneurones (Araki, Eccles & Ito, 1960). To make our results compatible with data derived from intracellular recordings the effect of a single volley in the lateral gastrocnemius-soleus nerve on medial gastrocnemius excitability was recorded and the latency of all sural effects was measured relative to the onset of this monosynaptic facilitatory effect. To calculate the absolute latency from cord entry of sural effects on medial gastrocnemius motoneurones, 0 5 ms was added to the latency as measured above to allow for the mean lateral gastrocnemius-soleus intracord conduction time and a single synaptic delay at medial gastrocnemius motoneurones (Brock, Coombs & Eccles, 1952; see also Fig. 11 of Araki et al. 1960). This corrected figure has been used in all results. RESULTS
In order to determine the percentage of the medial gastrocnemius motoneurone
pool activated in these experiments, the area of the antidromic medial gastrocnemius volley was measured in the distal end of the cut ventral root (Clamann, Gillies & Henneman, 1974). The area of the control reflex was 10-15% of the area of the antidromic volley which, allowing for the non-linearity introduced by the different fibre diameters (see Discussion), indicates that the unconditioned reflexes involved 15-25 % of the most excitable motoneurones of the medial gastrocnemius pool. These results are based on excitability cycles for the medial gastrocnemius mono-
synaptic reflex of eleven cats, conditioned by sural stimuli with amplitudes of 1-5 (8 cycles), 3 (8 cycles), 5 (8 cycles), 10 (5 cycles) and 40 (5 cycles) times the nerve threshold. The ipsilateral motor cortex had been removed several months prior to the experiment in six cats and in the remainder the nervous system was intact prior to the acute spinalization. The results were similar for both types preparation and have been presented as a single group. The usual change in medial gastrocnemius reflex excitability following strong sural shocks (10-40 times threshold) was triphasic; this consisted of an early period of facilitation followed by inhibition of a similar magnitude and finally a prolonged facilitation of smaller amplitude than the earlier effects. This pattern was observed in 5 out of 7 excitability cycles. At lower stimulus strengths (1.5-5 times threshold) only the earlier diphasic changes were found (9 out of 10 cycles). The early sural faciliation was first present following stimuli of 1-5-3 times the nerve threshold and was maximal with stimuli 3-5 times threshold (Fig. 1). At its peak, this effect increased the amplitude of the medial gastrocnemius reflex by a
J. G. COLEBATCH AND J. D. GILLIES mean of 86 % (range: 37-120 %). In each animal the relative effectiveness of this early facilitation was compared with the maximum facilitation following a single stimulus applied to the nerve to lateral gastrocnemius-soleus. The maximum amplitude of the early sural facilitation was from 60-90 % of the monosynaptic facilitation obtainable from lateral gastrocnemius-soleus. Its mean latency from cord entry was 2*8 ms (range: 2*5-3*1 ms). The excitatory effect lasted a mean period of 4'0 ms 404
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405 MOTONEURONAL EXCITABILITY (range: 2*9-4*9 ms) after which a period of inhibition followed (mean duration 16 ms, range: 9-24 ms). This inhibitory effect involved fibres first recruited by stimuli with amplitudes between 1*3 and 3 times the nerve threshold but the maximum effect was not observed until stimulation levels 40 times threshold were used. In all cases this inhibition was, with stimulation at 40 times threshold strength, sufficient at its peak to abolish completely the monosynaptic medial gastrocnemius reflex. The later excitation, which followed the period of inhibition, was seen with sural stimuli of 10-40 times the nerve threshold. The effect was always smaller than the earlier facilitation (mean reflex increase, 43 %, range: 15-83 %) but more prolonged (mean duration, 50 ms, range: 40-70 ms). Triphasic excitability changes were not invariably found. Two cats showed different patterns of excitability change. In one, the latency of the inhibitory phase was increased and inhibition did not follow activity in low threshold fibres. At the latency when inhibition usually began, there was in this animal a second peak of facilitation (Fig. 2). This facilitation first appeared with stimulus levels of 1P5 times the nerve threshold but did not reach its peak until stimuli of 10 times threshold were used. This preparation also displayed unusually pronounced flexor responses following sural volleys and had marked ankle clonus. The second atypical preparation showed a contrasting pattern. Late excitation was absent and the phase of inhibition was unusually prolonged with a duration of 29 ms. The motor cortex of this cat had been removed 2 months prior to the experiment, but since these changes were not found in the other five similar preparations this unusual pattern could not be attributed to the antecedent lesion. DISCUSSION
Single volleys in the sural nerve produced a sequence of excitability changes in medial gastrocnemius. Sural afferents recruited by stimuli with amplitudes of less than 1*5 times the nerve threshold caused no excitability changes; this 'ineffectiveness', at a segmental level, of activity in the largest fibres of the sural nerve has been previously reported (Rosenberg, 1970). Afferents recruited by stimuli with Fig. 1. An example of the changes in medial gastrocnemius excitability most commonly found after sural nerve stimulation. Each point represents one observation of integrated reflex amplitude. The upper panel of A shows the triphasic excitability change which followed high amplitude (40 times threshold (T)) stimulation of the sural nerve. The control, immediately below, indicates the discharge which was evoked by the sural stimulus alone and must be subtracted from the upper panel to obtain the actual medial gastrocnemius excitability changes. B shows more detail of the early changes (note faster time base). A sural stimulus with an amplitude 1-5 times nerve threshold level caused no alteration in medial gastrocnemius excitability but a stimulus of 3 times nerve threshold caused a 4 ms period of excitation as well as later inhibition which is present at the end of the record. Control records (not reproduced here) indicated that these sural volleys alone evoked no discharge. The arrows show the time when monosynaptic excitation began after stimulation of the nerve to lateral gastrocnemius-soleus and are used to calculate the latencies of the effects (see text). Some points have been retouched.
J. G. COLEBATCH AND J. D. GILLIES 406 amplitudes of between 1P5 and 3 times the nerve threshold have both excitatory and inhibitory effects. The earliest segmental effect of sural stimuli is facilitation, with a latency (mean, 2-8 ms from cord entry) consistent with a trisynaptic linkage. High threshold (10-40 times nerve threshold) fibres also have both inhibitory and, later, excitatory actions. Despite the high stimulus levels, the latency of even the .
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407 MOTONEURONAL EXCITABILITY late effects is consistent with myelinated fibre activity. With the exception of the lowest threshold fibres, afferents with excitatory and inhibitory projections are present throughout the myelinated fibre spectrum of the sural nerve. The motoneurones of a motor pool appear to have a relatively fixed order of recruitment in response to changes in synaptic excitation (Henneman, Somjen & Carpenter, 1965). If this is the case, the first recruited must be the cells most used by the animal during movements. Ths makes the reflex connections of these cells of particular interest. However, these cells are also the smallest present in the pool which means that, for medial gastrocnemius, their diameters approximate those of large y motoneurones (Burke, Strick, Kanda, Kim & Walmsley, 1977). Intracellular study of these small motoneurones is likely to be almost as difficult as it is for y motoneurones, which are only occasionally entered and deteriorate rapidly (Eccles, Eccles, Iggo & Lundberg, 1960). The excitability cycle technique, which consists of recording the change in recruitment following an homonymous la volley, offers a simple method to examine specifically the excitability changes of a population of these small cells. Results obtained with this technique may therefore differ from commonly reported intracellular records. However, since the voltage generated by an axon is proportional to the square of its diameter (Clamann & Henneman, 1976), there is not a linear relationship between the percentage of the motor pool discharged and the amplitude of the integrated reflex. As a consequence the percentage increase in the integrated reflex amplitude is greater than the percentage of new motoneurones recruited. It is possible to calculate, given the fibre distribution reported by Boyd & Davey (1968), that the control reflex involved 15-25 % of the total pool and that to double its amplitude required only about a 70 % increase in the number of fibres discharging. Although well suited to studying this part of the motoneurone pool, excitability changes shown with this technique may not occur post-synaptically. The size of the monosynaptic reflex is a measure of the excitability of the pathway from the la fibres to the motoneurones, so that changes in the excitability of either of these two elements will cause alterations in the size of the test reflex. Evidence that these changes do occur post-synaptically comes from intracellular recordings from medial gastrocnemius motoneurones. These have shown that both early and late depolarizations, as well as hyperpolarization occur in some medial gastrocnemius motoneurones following sural stimuli (Wilson, 1963; Engberg, 1964; Burke et al. 1970; Fig. 2. An example of an exception to the more common diphasic pattern of the early excitability changes following sural volleys. In this animal stimulation of the sural nerve at two times the threshold level (T) (caused an initial peak of excitation followed by a
further period of facilitation. This later period of facilitation contrasted with the more frequent presence of inhibition following a short initial period of excitation (Fig. 1). An increase in the stimulus intensity to 10 times the nerve threshold caused the early period of facilitation to be increased slightly (after the control discharge is subtracted) whereas the following excitation is markedly increased. These findings suggest that a facilitatory pathway, more potent in this animal, is present in addition to the inhibitory one which usually is dominant. The arrows indicate the time when facilitation following stimulation of the lateral gastrocnemius-soleus nerve began on comparable records and are used to derive latencies (see text). Some of the points have been retouched.
J. G. COLEBATCH AND J. D. GILLIES Rosenberg, 1970; see below). Hyperpolarization of la terminals follows sural volleys with a latency similar to that of the late period of facilitation (Rudomin, Nunez, Madrid & Burke, 1974). A contribution from this source is unlikely, however, as the fibres responsible for the hyperpolarization have a low threshold, unlike those 408
responsible for the late facilitation described here. Facilitatory effects of sural volleys on extensor motoneurones have been shown previously. Bernhard (1947), using the excitability cycle technique, showed a period of facilitation following inhibition of gastrocnemius after sural volleys. The latency of this change was compatible with the late facilitation shown here. No early facilitation was noted. Wilson (1963) found, with intracellular recording from gastrocnemius-soleus cells, that sural volleys frequently caused short latency e.p.s.p.s. These latencies ranged from 2*5 to 5 ms and the thresholds of the fibres responsible ranged from 1-2 to 2*2 times the nerve threshold. He also found late depolarizations in response to stimuli of much higher amplitudes. Our results are in general agreement with these observations, as our briefer latencies would be biased towards the earliest facilitation occurring in the group of motoneurones. Our results are also consistent with those of Burke et al. (1970) who examined the nature of the sural projection to different parts of the triceps surae motor pool. Half of the motoneurones innervating type S motor units, which in general have the smallest sizes (Burke, 1967), had early e.p.s.p.s followed by a later i.p.s.p. An additional advantage of recording reflex amplitude changes to establish neural connections is that an estimate of the relative effectiveness of two projections to a population of motoneurones can be made, unaffected by the sampling bias of intracellular recording. The early facilitation was found to augment the reflex area by 60-90 % of that caused by a heteronymous la volley from lateral gastrocnemiussoleus. The early facilitatory sural projection must therefore be regarded both as a potent and widespread segmental facilitatory input to medial gastrocnemius motoneurones. Cutaneous fibres were included among the 'flexion reflex afferents' grouping of Holmqvist & Lundberg (1961). Their inclusion was not entirely consistent with earlier observations (Hagbarth, 1952) and subsequent evidence derived from intracellular recordings (Wilson, 1963; Engberg, 1964; Hongo, Jankowska & Lundberg, 1966, 1969) led to the separation of low threshold cutaneous fibres from the 'flexion reflex afferents', which continued to include high threshold fibres (Hongo et al. 1969). Our results for the sural projection indicate that high threshold fibres, as well as low threshold fibres, have excitatory actions in addition to the accepted inhibitory effects. These excitability changes are not necessarily typical of other cutaneous projections, although Wilson (1963) also found late depolarizations after stimulating the superficial peroneal nerve. The early facilitatory change, attributed to relatively low threshold sural afferents, was always present in these experiments. In contrast, the later inhibitory and excitatory phases were not seen in every animal, and were sometimes replaced by activity of the opposite type. These observations suggest that these later phases normally represent the net result of both excitatory and inhibitory projections converging upon these motoneurones and either acting simultaneously or alternately. Such parallel pathways have been shown for projections to flexor motoneurones
409 MOTONEURONAL EXCITABILITY (Holmqvist & Lundberg, 1961) as well as in walking cats (Forssberg et al. 1975; Duysens & Pearson, 1976). This arrangement would also explain the change from late facilitation to inhibition following the administration of dial anaesthetic (Hagbarth & Naess, 1950). In these medial gastrocnemius motoneurones, the typical excitability changes following sural volleys is a triphasic one, with inhibition preceded and followed by facilitation. The early period of facilitation at least has such a potency and is so widespread that further investigation should attempt to delineate in more detail its characteristics and role in movement. Future investigation should also attempt to show if these changes are typical of this part of the motor pool of other extensor muscles following activity in cutaneous nerves. REFERENCES
ARAKI, T., ECCLES, J. C. & ITO, M. (1960). Correlation of the inhibitory post-synaptic potential of motoneurones with the latency and time course of monosynaptic reflexes. J. Physiol. 154, 354-377. BERNHARD, C. G. (1947). Slow cord potentials correlated to reciprocal functions. Acta physiol. scand. 14, suppl. 47, 6. BOYD, I. A. & DAVEY, M. R. (1968). Composition of Peripheral Nerves, p. 28. Edinburgh: Livingstone. BROCK, L. G., COOMBS, J. S. & ECCLES, J. C. (1952). The recording of potentials from motoneurones with an intracellular electrode. J. Physiol. 117, 431-460. BURKE, R. E. (1967). Motor unit types of cat triceps surae muscle. J. Physiol. 193, 141-160. BURKE, R. E., JANKOWSKA, E. & TEN BRUGGENCATE, G. (1970). A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae. J. Physiol. 207, 709-732. BURKE, R. E., STRICK, P. L., KANDA, K., KIM, C. C. & WALMSLEY, B. (1977). Anatomy of medial gastrocnemius and soleus motor nuclei in cat spinal cord. J. Neurophysiol. 40, 667-680. CLAMANN, H. P., GILLIES, J. D. & HENNEMAN, E. (1974). Effects of inhibitory inputs on critical firing level and rank order of motoneurones. J. Neurophysiol. 37, 1350-1360. CLAMANN, H. P., GILLIES, J. D., SKINNER, R. D. & HENNEMAN, E. (1974). Quantitative measures of output of a motoneurone pool during monosynaptic reflexes. J. Neurophysiol. 39, 1328-1337. CLAMANN, H. P. & HENNEMAN, E. (1976). Electrical measurement of axon diameter and its use in relating motoneuron size to critical firing level. J. Neurophysiol. 39, 844-851. COLEBATCH, J. G. & GILLIES, J. D. (1977). Effects on cat medial gastrocnemius motoneurones of segmental input from the sural nerve. Proceedings of the Australian Physiological and Pharmacological Society. 8, 56P. CREED, R. S., DENNY-BROWN, D., ECCLES, J. C., LIDDELL, E. G. T. & SHERRINGTON, C. S. (1932). Reflex Activity of the Spinal Cord, repr. 1972. Oxford: Oxford University Press. DUYSENS, J. & PEARSON, K. G. (1976). The role of cutaneous afferents from the distal hindlimb in the regulation of the step cycle of thalamic cats. Exp. Brain Res. 24, 245-255. ECCLES, J. C., ECCLES, R. M., IGGO, A. & LUNDBERG, A. (1960). Electrophysiological studies on gamma motoneurones. Acta physiol. scand. 50, 32-40. ECCLES, R. M. & LUNDBERG, A. (1959). Synaptic actions in motoneurones by afferents which may evoke the flexion reflex. Archs ital. Biol. 97, 199-221. ENGBERG, I. (1964). Reflexes to foot muscles in the cat. Acta physiol. scand. 62, Suppl. 235. FORSSBERG, H., GRILLNER, S. & ROSSIGNOL, S. (1975). Phase dependent reflex reversal during walking in chronic spinal cats. Brain Res. 85, 103-107. HAGBARTH, K.-E. (1952). Excitatory and inhibitory skin areas for flexor and extensor motoneurones. Acta physiol. scand. 26, supply. 94. HAGBARTH, K.-E. & NAESS, K. (1950). Reflex effects of tetanic stimulation of different afferent fibre-systems in the hind limb of the cat. Acta physiol. scand. 21, 336-361. HENNEMAN, E., SOMJEN, G. & CARPENTER, D. 0. (1965). Excitability and inhibitibility of motoneurons of different sizes. J. Neurophysiol. 28, 599-620.
410
J. G. COLEBATCH AND J. D. GILLIES
HOLMQVIST, B. & LUNDBERG, A. (1961). Differential supraspinal control of synaptic actions evoked by volleys in the flexion reflex afferents in alpha motoneurones. Acta phy8iol. 8cand. 54, suppl. 186. HONGO, T., JA~xowsKA, E. & LUNDBERG, A. (1966). Convergence of excitatory and inhibitory action on interneurones in the lumbosacral cord. Exp. Brain Res. 1, 338-358. HoNGo, T., JwowsKA, E. & LUNDBERG, A. (1969). The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones. Exp. Brain Res. 7, 365-391. LLOYD, D. P. C. (1943). Neuron patterns controlling transmission of ipsilateral hind limb reflexes in cat. J. Neurophysiol. 6, 293-315. ROSENBERG, M. E. (1970). Synaptic connexions of alpha extensor motoneurones with ipsilateral and contralateral cutaneous nerves. J. Physiol. 207, 231-255. RUDOMIN, P., NuNEz, R., MADRID, J. & BURKE, R. E. (1974). Primary afferent hyperpolarisation and presynaptic facilitation of la afferent terminals induced by large cutaneous fibers. J. Neurophysiol. 37, 413-429. SHERRINGTON, C. S. (1910). Flexion-reflex of the limb, crossed extension-reflex, and reflex stepping and standing. J. Physiol. 40, 28-121. SHERRINGTON, C. S. (1947). The Integrative Action of the Nervous System, 2nd edn. New Haven: Yale University Press. WILSON, V. J. (1963). Ipsilateral excitation of extensor motoneurones. Nature, Lond. 198, 290291.
